Polynitrogen compounds are of significant interest as high energy density materials (HEDM), particularly for propulsion or explosive applications. In spite of numerous theoretical studies predicting that certain all-nitrogen compounds might be stable, to date, experimental studies aimed at their actual synthesis have been unsuccessful.
Presently, only two homoleptic polynitrogen species are known which can be prepared on a macroscopic scale: dinitrogen, N.sub.2, which was independently isolated in pure form from air in 1772 by Rutherford, Scheele, and Cavendish; and the azide anion, N.sub.3.sup.-, discovered in 1890 by Curtius. Other species, such as N.sub.3., N.sub.3.sup.+, and N.sub.4.sup.+ have been observed only as free gaseous or matrix-isolated ions or radicals. In spite of the extensive theoretical studies hypothecating that species such as N.sub.4 (T.sub.d), N.sub.8, (O.sub.h), N(N.sub.3).sub.2.sup.-, N(N.sub.3).sub.3, and N(N.sub.3).sub.4.sup.+, and N.sub.5.sup.+ (Prykko and Runeberg, T. Mol. Struct. Trichem 1991, 234, 279) are vibrationally stable, the lack of a single successful synthesis of a new species on a macroscopic scale is surprising and may reflect the great experimental difficulties resulting from their high endothermicities, which give rise to instability and unpredictable explosiveness.
The high energy content of polynitrogen candidates stems, at least in part, from the N--N single and double bonds they possess, with average bond energies of 38.2 and 99.9 kcal/mol, respectively. These bond energies are significantly less than the N.sub.2 triple bond energy of 228 kal/mol. Therefore, any transformation of a polynitrogen compound to N.sub.2 molecules is accompanied by a very large energy release and any new metastable polynitrogen compound will be isolable and manageable only if it possesses a sufficiently large energy barrier to decomposition.